msm: kgsl: Avoid duplicate entries in snapshot

GMU related registers and debug bus blocks are duplicated in the
gpu snapshot. Fix this by avoiding the duplicate dump in
kgsl_device_snapshot(). a6xx_snapshot internally calls
a6xx_snapshot_gmu to dump GMU state. Don't invoke a6xx_snapshot_gmu
explicitly after a6xx_snapshot to avoid duplicate entries in
snapshot.

Change-Id: I03453f6ca1f4d706d9a6fe45d2e3be9954dd3db0
Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>